The present invention relates to a printer having a paper transporting device and a printing head which is equipped with a plurality of closely spaced, electrically energizable printing elements arranged in a row, wherein the printing elements having odd and even ordinal numbers, or vice versa, are alternatingly energizable with variable energy corresponding to the gray values of the image information so as to represent the gray values by different size image dots.
Such a printer, in the form of a thermal printer, is disclosed in DE-A No. 3,309,328. For printing of a matrix line, the heating elements of the thermal printing head are energized in two consecutive time periods, with adjacent heating elements never being traversed simultaneously by heating current. In this way it is prevented that adjacent image dots run into one another due to thermal crosstalk. By varying the heating currents, the size of the image dots can be varied. The range over which the image dot size can be varied is limited by the distance between the heating elements. In presently customary thermal printing heads including an arrangement of 12 to 16 heating elements per millimeter, the possible variation in image dot size is relatively small with respect to high contrast gray and color tone gradations.
The principle of dot-matrix printing is sufficiently known from the printing art. There, too, different areal expanses of the matrix dots which are gradated as to their tone value, simulate a half-tone effect. This is based on the fact that the human eye is unable to discern the individual matrix dots in their small size and density. This limited resolution capability of the human eye also forms the basis for the matrix sizes employed in the printing art, for example, sixty lines per centimeter, i. e. six lines per millimeter, for a higher standard quality, with the matrix dots being equidistantly arranged in the direction of the line (Erhard D. Stiebner et al, "Drucktechnik heute" [Printing Technology Today], 1985, published by Verlag F. Bruckmann KG, Munich, ISBN 3-7654-1951-6, page 84).